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A citadel is rising on the banks of the Tigris. There, on the river’s western side, the United States is building the world’s largest embassy. The land beneath it was once a riverside park. What sits atop today is a massive, fortified compound. Encircled by blast walls and cut off from the rest of Baghdad, it stands out like the crusader castles that once dotted the landscape of the Middle East. Its size and scope bring into question whether it is even correct to call this facility an “embassy.” Why is the United States building something so large, so expensive, and so disconnected from the realities of Iraq? In a country shattered by war, what is the meaning of this place?

For security reasons, many details about the embassy’s design and construction must remain classified. But the broad outline of its layout says a lot about one of America’s most important architectural projects. Located in Baghdad’s 4-square-mile Green Zone, the embassy will occupy 104 acres. It will be six times larger than the U.N. complex in New York and more than 10 times the size of the new U.S. Embassy being built in Beijing, which at 10 acres is America’s second-largest mission. The Baghdad compound will be entirely self-sufficient, with no need to rely on the Iraqis for services of any kind. The embassy has its own electricity plant, fresh water and sewage treatment facilities, storage warehouses, and maintenance shops. The embassy is composed of more than 20 buildings, including six apartment complexes with 619 one-bedroom units. Two office blocks will accomodate about 1,000 employees. High-ranking diplomats will...